1. Field of the Invention
The present invention relates to a lock-up clutch mechanism for a torque converter, used in an automatic transmission of a vehicle and the like, and more particularly, it relates to an improvement in a friction plate of a lock-up clutch mechanism.
2. Related Background Art
In recent years, in lock-up clutches for torque converters used in automatic transmissions, in order to enhance reduction of fuel consumption, lock-up has been started from an area where a vehicle speed is low. In this case, slip control of the lock-up clutch has been performed to absorb vibration of an engine during a low speed operation. That is to say, a pressure difference across a piston plate is reduced, so that a clutch facing is contacted with a front cover at a low pressure to absorb torque fluctuation of the engine.
Further, at a speed range greater than a middle speed, the clutch facing is contacted with the front cover at a higher pressure difference across the piston plate, thereby achieving complete lock-up. In an initial engagement condition and in the slop control, there has been requested to provide a friction material having an excellent μ-V property and good heat-resistance and having an ability which friction material does not generate judder in a condition that the lock-up clutch is being slipped. Further, there has been requested to provide a friction material having high coefficient of static friction and which does not generate slip in a full engagement condition.
Further, the piston plate has a tendency in which the piston plate is flexed so that an inner diameter portion thereof is shifted toward the front cover as the pressure difference is increased. Thus, in order to obtain uniform surface pressure both in case of small pressure difference and great pressure difference, it is desirable that shapes of friction materials be differentiated in a radial direction.
To satisfy such requirements, it is desirable that friction materials having different shapes and/or properties be stuck in adjacent to each other in a radial direction so as to provide optimum friction surfaces under respective conditions. An example that friction materials having different shapes and/or properties are stuck is disclosed in Japanese Patent Application Laid-open No. S57-140920 (1982).
However, as described in the Japanese Patent Application Laid-open No. S57-140920, when different friction materials are aligned in the radial direction, in order to stick the friction materials concentrically with high accuracy, a complicated sticking machine is required, and, if the friction materials are aligned before the sticking, since the friction materials are apt to be peeled easily, there arises a problem regarding workability.